Diabetic kidney disease is increasing in prevalence and is associated with significant morbidity and mortality. Health disparities exist in the progression of diabetic kidney disease, with minorities being more affected even when adjusting for treatment, glycemic and hypertensive control, and medical coverage. Secondary prevention of the progression of diabetic kidney disease is hindered by a lack of easily modifiable risk factors. Based on animal and observational studies, vitamin D deficiency is potentially a novel, modifiable risk factor that may interrupt or delay the progression of diabetic kidney disease through direct effects as well as by helping to ameliorate kidney disease risk factors, such as hyperglycemia, hypertension and inflammation. In addition, based on minorities having a higher prevalence of vitamin D deficiency, it may also potentially impact the differential progression of diabetic kidney disease in minorities. However, clinical trials evaluating the impact of vitamin D supplementation on diabetic kidney disease are lacking. Thus, this pilot study has the following specific aims: 1) to evaluate the impact of vitamin D supplementation (vitamin D3 at 400 IU/d versus supplementation of 4000 IU/d) on the proportion of African Americans with progression of albuminuria over 12 months and 2) to identify whether kidney disease risk factors such as blood pressure and glycemic control mediate the impact of vitamin D supplementation on the progression of albuminuria over 12 months. We will conduct a randomized, double-blind, placebo-controlled trial among 102 African American adults ?=18 years old) with vitamin D deficiency, previously diagnosed type 2 diabetes and stage 1 or 2 chronic kidney disease receiving usual care for their diabetes. After a 3 month run-in period of vitamin D supplementation, we will follow participants for an additional 12 months. This pilot will provide relevant information to be used to design a larger randomized, controlled trial of vitamin D supplementation on the progression of diabetic kidney disease. Based on current health disparities and vitamin D's biologic mechanisms of action, vitamin D deficiency has the possibility of being a novel, modifiable risk factor that could be added to usual care to decrease health disparities by delaying the progression of diabetic kidney disease. Since much information is unknown regarding the role of Vitamin D in diabetic kidney disease due to the lack of human studies, this pilot study is an important first step towards evaluating the potential impact of this intervention. PUBLIC HEALTH RELEVANCE: Diabetic kidney disease is increasing in prevalence and is responsible for considerable morbidity and mortality. Vitamin D deficiency is potentially a novel, modifiable risk factor that could delay the progression of diabetic kidney disease. Minorities have been shown to have a higher prevalence rate of vitamin D deficiency and are more affected by health disparities that exist in the progression of kidney disease. Due to a lack of clinical trials and human studies, the role of vitamin D in the delaying the progression of early stage diabetic kidney disease is unknown. This project is an attempt to determine the impact of vitamin D supplementation on the progression of early stage diabetic kidney disease.